The 27 year old plays a warrior in the film, which chronicles the devastating volcanic eruption which engulfed the ancient city, and he admits the four-hour "gladiator boot camp" training and strict diet regiment he had to sign up for to perfect his look in the movie was difficult to deal with.
He tells the New York Post, "The diet... was very tough. Having to cut down to under 2,000 calories a day is not fun for anyone.
"On top of that we had to do lots and lots of fighting... Learning new swordsmanship with two swords was a whole new practice."
However, the rigorous routine had an added bonus for the English star - he returned to the set of Game of Thrones a more skilled sword fighter.
He continues, "I came back into Thrones quite a lot better at sword fighting, which is good because there is quite a lot this year."